This invention generally relates to medical devices, and particularly to balloon catheters, stent covers, and vascular grafts.
In percutaneous transluminal coronary angioplasty (PTCA) procedures, a guiding catheter is advanced until the distal tip of the guiding catheter is seated in the ostium of a desired coronary artery. A guidewire, positioned within an inner lumen of an dilatation catheter, is first advanced out of the distal end of the guiding catheter into the patient""s coronary artery until the distal end of the guidewire crosses a lesion to be dilated. Then the dilatation catheter having an inflatable balloon on the distal portion thereof is advanced into the patient""s coronary anatomy, over the previously introduced guidewire, until the balloon of the dilatation catheter is properly positioned across the lesion. Once properly positioned, the dilatation balloon is inflated with liquid one or more times to a predetermined size at relatively high pressures (e.g. greater than 8 atmospheres) so that the stenosis is compressed against the arterial wall and the wall expanded to open up the passageway. Generally, the inflated diameter of the balloon is approximately the same diameter as the native diameter of the body lumen being dilated so as to complete the dilatation but not overexpand the artery wall. Substantial, uncontrolled expansion of the balloon against the vessel wall can cause trauma to the vessel wall. After the balloon is finally deflated, blood flow resumes through the dilated artery and the dilatation catheter can be removed therefrom.
In such angioplasty procedures, there may be restenosis of the artery, i.e. reformation of the arterial blockage, which necessitates either another angioplasty procedure, or some other method of repairing or strengthening the dilated area. To reduce the restenosis rate and to strengthen the dilated area, physicians frequently implant an intravascular prosthesis, generally called a stent, inside the artery at the site of the lesion. Stents may also be used to repair vessels having an intimal flap or dissection or to generally strengthen a weakened section of a vessel. Stents are usually delivered to a desired location within a coronary artery in a contracted condition on a balloon of a catheter which is similar in many respects to a balloon angioplasty catheter, and expanded to a larger diameter by expansion of the balloon. The balloon is deflated to remove the catheter and the stent left in place within the artery at the site of the dilated lesion. Stent covers on an inner or an outer surface of the stent have been used in, for example, the treatment of pseudo-aneurysms and perforated arteries, and to prevent prolapse of plaque. Similarly, vascular grafts comprising cylindrical tubes made from tissue or synthetic materials such as DACRON may be implanted in vessels to strengthen or repair the vessel, or used in an anastomosis procedure to connect vessels segments together.
In the design of catheter balloons, balloon characteristics such as strength, flexibility and compliance must be tailored to provide optimal performance for a particular application. Angioplasty balloons preferably have high strength for inflation at relatively high pressure, and high flexibility and softness for improved ability to track the tortuous anatomy and cross lesions in the uninflated state. The balloon compliance is chosen so that the balloon will have a desired amount of expansion during inflation. Compliant balloons, for example balloons made from materials such as polyethylene, exhibit substantial stretching upon application of internal pressure. Noncompliant balloons, for example balloons made from materials such as PET, exhibit relatively little stretching during inflation, and therefore provide controlled radial growth in response to an increase in inflation pressure within the working pressure range.
For many applications, intravascular catheter balloons should be substantially noncompliant once expanded to a working diameter. Further, catheter balloons should also be formed from relatively strong materials in order to withstand the pressures necessary for various procedures without failing. Typically, such characteristics require the use of a material that does not stretch appreciably, which consequently necessitates that the balloon material be folded around the catheter shaft prior to inflation. However, it can be desirable to employ balloons that are not folded prior to inflation, but which are instead expanded to the working diameter from a generally cylindrical or tubular shape having a nominal diameter that conforms to the catheter shaft. Such designs may be used for formed-in-place angioplasty balloons and stent delivery balloons. Prior art formed-in-place balloons have suffered from problems such as insufficient strength, poor control over expansion, and significantly complicated processing during catheter manufacturing.
It would be a significant advance to provide a catheter balloon, and other expandable members such as stent covers, and vascular grafts, with improved processing and expansion characteristics.
This invention is directed to medical devices, and particularly intracorporeal devices for therapeutic or diagnostic uses, having at least a component formed of ultrahigh molecular weight polyolefin (UHMW polyolefin). In a presently preferred embodiment, the UHMW polyolefin is an ultrahigh molecular weight polyethylene (UHMW polyethylene). A presently preferred embodiment is directed to UHMW polyolefin which is microporous, and having a node and fibril microstructure comprising nodes interconnected by fibrils.
One embodiment of the invention comprises an expandable member such as a balloon for an intraluminal catheter, formed at least in part of the UHMW polyolefin, such as UHMW polyethylene. In another embodiment of the invention, a stent delivery system comprising a balloon catheter and a stent mounted on the balloon has a component, such as the catheter balloon or a stent cover, which is formed at least in part of the UHMW polyolefin, such as UHMW polyethylene. Another embodiment of the invention comprises a vascular graft formed at least in part of the UHMW polyolefin, such as UHMW polyethylene. The terminology vascular graft as used herein should be understood to include grafts and endoluminal prostheses, such as those surgically attached to vessels, as for example in vascular bypass or anastomosis, or implanted within vessels, as for example in aneurysm repair or at the site of a balloon angioplasty or stent deployment. Although discussed below primarily in terms of a balloon catheter having a balloon formed of UHMW polyethylene, the invention should be understood to include other medical devices and particularly intracorporeal devices for a therapeutic or diagnostic purpose, such as stent covers and vascular grafts formed of UHMW polyolefin, such as UHMW polyethylene. Additionally, although discussed primarily in terms of UHMW polyethylene, it should be understood that the invention applies as well to UHMW polyolefins in general, and to other materials having a node and fibril microstructure such as polypropylene, nylon, and expanded polytetrafluoroethylene.
The UHMW polyethylene has a molecular weight which is higher than the molecular weight of high molecular weight polyethylenes, and which is about 2 million to about 10 million grams/mole, preferably about 3 million to about 6 million grams/mole. Unlike high molecular weight polyethylenes, which generally have a molecular weight of about 400,000 to about 600,000 grams/mole, the UHMW polyethylene is difficult to melt process. Balloons formed from this material exhibit compliant expansion at relatively low strains and exhibit substantially less compliance at higher strains.
The node and fibril structure of the UHMW polyethylene causes it to exhibit essentially compressible deformation at relatively small strains, with a low Young""s modulus in tension for the compressed material. At high strains, the UHMW polyethylene balloons of the invention preferably exhibit low compliance due to rearrangement in the microstructure. Embodiments of the invention suited to intravascular applications preferably exhibit compliant radial expansion of about 70% to about 450%, and more particularly 100% to about 400%, of the uninflated diameter, at pressures up to about 6 to about 8 atm. Once expanded, the balloons exhibit relatively low compliance at pressures above about 8 atm and can have a burst pressure of at least about 18 atm. In one embodiment, the UHMW polyethylene exhibits microstructural rearrangement and the balloon exhibits low compliance in the working pressure range with a radial expansion of about 5% to about 20%, and preferably less than about 15% of the uninflated diameter of the balloon, at inflation pressures of about 6 atm to about 18 atm. In one embodiment, the UHMW polyethylene exhibits a negative Poisson ratio. For stent delivery applications, the UHMW polyethylene preferably has a foam-like compressible state at low strains so that the stent can be crimped onto the balloon with good retention.
One aspect of the invention is directed to a noninflated balloon comprising ultrahigh molecular weight polyolefin in a compressed configuration having a reduced porosity relative to the ultrahigh molecular weight polyolefin in a noncompressed configuration. In one embodiment, the balloon formed of UHMW polyolefin, such as UHMW polyethylene, is compressed in the radial direction of the balloon, to provide a reduced profile medical device component. In another embodiment, the balloon formed of UHMW polyolefin, such as UHMW polyethylene, is compressed in the axial direction of the balloon, to provide reduced axial lengthening during radial expansion of the medical device component formed therefrom.
In another embodiment, the UHMW polyolefin, such as UHMW polyethylene, is compressed before being formed into a balloon, to provide a balloon exhibiting improved stress/strain curve response. The UHMW polyolefin compressed before being formed into a balloon is preferably compressed in the direction of the fibrils of the material, i.e., in the direction of the deformation which imparted the node and fibril structure to the material, while the thickness of the material is held constant. However, in an alternative embodiment, the UHMW polyolefin is compressed perpendicular or substantially perpendicular to the direction of the fibrils. The stress/strain curve response of the compressed UHMW polyolefin is characterized by a sudden increase of stress at increased strain, which allows for the construction of a balloon having desired expansion characteristics with a compliant radial expansion at initial inflation pressures and relatively low compliance at higher inflation pressures. Thus, the compressed UHMW polyethylene exhibits an improved substantial or compliant expansion upon inflation to an internal pressure within a first pressure range, and substantially less expansion within a second pressure range higher than the first pressure range. The compressed material provides a wingless balloon that can provide the desired radial compliance characteristics without excessive balloon-length increase or shortening during inflation of the balloon. The shape of the stress/strain curve of the resulting compressed UHMW polyethylene material will depend on factors such as porosity of the material before compression, compression conditions, fibril length, node size and node aspect ratio, ram extrusion conditions, material deformation or orientation conditions, heat set conditions, and the balloon construction characteristics such as winding angle. The compressed UHMW polyethylene is particularly useful for a stent deploying balloon, due to the lack of wings on the unexpanded balloon. In conventional stent deploying balloons, the folded balloon wings of the unexpanded balloon would unfold during inflation of the balloon, resulting in nonuniform expansion of the stent mounted on the unexpanded balloon.
Balloon catheters of the invention generally comprise an elongated shaft with at least one lumen and balloon formed of UHMW polyolefin such as UHMW polyethylene on a distal shaft section with an interior in fluid communication with the shaft lumen. The balloon catheters of the invention may be configured for a variety of uses, such as angioplasty or stent delivery. A stent delivery catheter employs a balloon having the characteristics of the invention to deploy the stent. Preferably, the oriented polyethylene exhibits a foam-like compressible state at low strains, facilitating crimping of the stent onto the balloon with improved stent retention. In accordance with the invention, the stent may be provided with a stent cover generally comprising a tubular sheath formed of the UHMW polyethylene and configured to be disposed on an outer and/or inner surface of the stent and implanted with the stent in the patient""s vessel.
Vascular grafts of the invention generally comprise a tubular body formed of the UHMW polyolefin such as UHMW polyethylene. The vascular graft is configured to be implanted in a patient, and may be used for a variety of procedures including anastomosis, bypass surgery, implantation within a vessel lumen to reduce restenosis, and aneurysm repair.
The invention also comprises methods of forming a medical device component such as a balloon, stent cover or vascular graft, from microporous polyolefin such as polyethylene having an oriented node and fibril structure. Generally, the method comprises the steps of compacting ultrahigh molecular weight polyethylene powder into a billet, deforming the compacted polyethylene to render the polyethylene microporous and to impart an oriented node and fibril structure to the polyethylene, and forming the medical device component from the polyethylene. Optionally, the powder can be sintered prior to deformation. Also optionally, the oriented polyethylene can be heat set. Preferably, a tubular medical device component such as a balloon may be formed by wrapping a film or sheet of the oriented polyethylene around a mandrel to form a tube and then heat fusing the polyethylene layers together, or by directly producing an oriented tubular member.
The medical devices such as catheter balloons, stent covers, and vascular grafts of the invention have improved performance due to the UHMW polyolefin such as UHMW polyethylene which is microporous, biocompatible, and biostable, and which has excellent mechanical properties. Further, UHMW polyethylene is more resistant to electron-beam (i.e., e-beam) degradation than expanded polytetrafluoroethylene (i.e., ePTFE) which degrades when exposed to e-beams, making e-beam sterilization more of an option than with ePTFE. Medical devices such as balloons of this invention can be expanded compliantly to their working diameter but exhibit substantially less compliance at greater pressures, providing control over expansion even at pressures suitable for conventional intravascular procedures such as angioplasty or stent delivery. Further, the formed-in-place balloons of the invention have sufficient strength to provide desired safety to conventional intravascular procedures. UHMW polyethylene also facilitates device manufacture, because the processing temperatures for polyethylene are relatively low, and the polyethylene can be readily attached with adhesives or heat bonded using tie layers to other device components. Thus, bonding to other device components is easier than with ePTFE.
These and other advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying exemplary drawings.